This invention relates to a method for analog-digital conversion, wherein an analog signal is sampled and a digital signal is output which corresponds to the analog signal. It further relates to a picture reproducing method which uses this method of analog-digital conversion.
In a machine for reproducing a picture, wherein an original picture is scanned and analog picture signals are produced, the analog picture signals are converted into digital picture signals by sampling according to a certain sampling frequency, these digital picture signals are stored into a memory, and then these digital picture signals are read out, converted into analog output picture signals, and then a reproduction picture is produced by using these analog reproduction picture signals. Thus the frequency range of the analog signal reproducible faithfully generally depends upon the sampling frequency, and, the higher is the sampling frequency, the finer detail can be reproduced. However, the higher is the sampling frequency, the greater is the capacity required for the memory, which involves a higher cost. Thus in practice limitations are imposed on the sampling frequency.
In order to avoid the abovementioned defects, another analog-digital conversion method for an analog signal has been proposed by the same inventor as the present invention. In this case, for example, an analog picture signal is converted into digital signals by sampling at a certain sampling rate corresponding to the analog picture signal by sampling sub-signals having a frequency of 3f.sub.0, which is three times as high as frequency as that of the sampling signal, which have a frequency of f.sub.0. The level of the sampling signal is obtained, as being: if the second one of the three consecutive sampling sub-signals is an extreme, i.e. the maximum or the minimum of them, then it; otherwise, the average of the three consecutive sampling sub-signals.
In this case, the frequency of the signal for sampling the analog signal is raised to be three times as high, but the output number of the sampling signals which are sent to the memory is the same, as in a conventional method. Thus the conventional memory can be used, without increasing its capacity.
However, this method has a disadvantage: that is, when noise components are mixed into an analog signal shown in FIG. 1a, as shown in FIG. 1b by jagged lines, the noise components are prone to be picked up as the level of the sampling signal at an extreme point or a point very near thereto.
That is, when the picture signal of FIG. 1b is sampled by the aforementioned method, in the sampling periods I, III, and IV of the sampling signal having the frequency f.sub.0 (sampling period 1/f.sub.0) the level of the sampling is the mean value of the three sampling sub-signals having the frequency 3f.sub.0 (sampling sub-period 1/3f.sub.0), and in the sampling periods II, V and VI of the sampling signal the level of the sampling signal is substantially the extreme of the three sampling sub-signals.
As shown in FIG. 1, for instance, in the sampling period V, since the relatively large noise component is included in the picture signal, although the sampling sub-signals simply increase or decrease and the average of these should be output, however, the noise component of the extreme is picked up as the output level of the sampling signal.